The central nervous system (CNS) of mammals employs many neuroactive peptides to effect specialized signaling within the brain and spinal cord including the neuroactive peptides somatostatin, cholecystokinin, VIP, Substance P, enkephalin, Neuropeptide Y (NPY), Neurotensin, TRH, CCK, and dynorphin. (see generally The Biochemical Basis of Neuropharmacology, Cooper, Bloom and Roth, 5th ed., Oxford University Press, New York, 1986). The careful elucidation of the complex signaling pathways, which operate in the CNS, has led to identification of specific receptors modulated by these neuroactive peptides presenting important therapeutic targets for various disorders associated with the CNS.
The N-methyl-D-aspartate (NMDA) receptor (NMDAR), is one such receptor that has been implicated in neurodegenerative disorders including stroke-related brain cell death, convulsive disorders, and learning and memory. NMDAR also plays a central role in modulating normal synaptic transmission, synaptic plasticity, and excitotoxicity in the central nervous system. The NMDAR is further involved in Long-term potentiation (LTP). LTP is the persistent strengthening of neuronal connections that underlie learning and memory (See Bliss and Collingridge, 1993, Nature 361:31-39).
Two general classes of glutamate receptors have been characterized in the central nervous system (CNS). They are the metabotropic glutamate receptors, which belong to the G-protein coupled receptor family of signaling proteins, and the ionotropic glutamate receptors (Muir and Lees, Stroke, 1995, 26, 503-513). The ionotropic class is further subdivided into the AMPA, kainate, and NMDA receptor subtypes by the selective ligands that activate them.
Ionotropic glutamate receptors contain a ligand-gated ion channel, which serves as a modulator of synaptic transmission. The NMDA receptor (NMDAR) is unique in that it requires both glutamate and glycine for activation and subsequent opening of the ion channel (Mothet et al., Proc. Nat. Acad. Sci., 2000, 97, 4926-4931). Recent studies have demonstrated that the glycine site can serve to modulate the activity of glutamate synaptic transmission at the NMDAR. Thus, both the glutamate and glycine sites can be utilized for modulation of NMDAR activity.
The NMDAR is activated by the binding of NMDA, glutamate (Glu), and aspartate (Asp). It is competitively antagonized by D-2-amino-5-phosphonovalerate (D-AP5; D-APV), and non-competitively antagonized by phenylcyclidine (PCP), and MK-801. Most interestingly, the NMDAR is co-activated by glycine (Gly) (Kozikowski et al., 1990, Journal of Medicinal Chemistry 33:1561-1571). The binding of glycine occurs at an allosteric regulatory site on the NMDAR complex, and this increases both the duration of channel open time, and the frequency of the opening of the NMDAR channel.
Recent human clinical studies have identified NMDAR as a novel target of high interest for treatment of depression. These studies conducted using known NMDAR antagonists CPC-101,606 and ketamine have shown significant reductions in the Hamilton Depression Rating Score in patients suffering with refractory depression. Although, the efficacy was significant, but the side effects of using these NMDAR antagonists were severe.
NMDA-modulating small molecule agonist and antagonist compounds have been developed for potential therapeutic use. However, many of these are associated with very narrow therapeutic indices and undesirable side effects including hallucinations, ataxia, irrational behavior, and significant toxicity, all of which limit their effectiveness and/or safety.
Further, 50% or more of patients with depression do not experience an adequate therapeutic response to known administered drugs. In most instances, 2 or more weeks of drug therapy are need before meaningful improvement is observed, as noted in an open-label study on pharmacological treatment of depression. (Rush et al, Am. J. Psychiatry 2006, 163: 1905). There currently is no single effective treatment for depression, anxiety, and other related diseases.
Thus, there remains a need for improved treatments of depression, anxiety and/or other related diseases with compounds that provide increased efficacy and reduced undesirable side effects.